a. Field of the Invention
The present invention relates to an apparatus and method for testing exhaust components. In particular the invention relates to an apparatus and method for testing the catalytic efficiency of exhaust components used in heavy-duty diesel engine exhaust systems.
b. Related Art
Modern exhaust after-treatment systems for diesel engines are becoming increasingly sophisticated. Typically, these systems consist of various components that are designed to reduce the amount of potentially harmful pollutants present in the exhaust gases. These pollutants include oxides of nitrogen such as nitrogen oxide (NO) and nitrogen dioxide (NO2) known collectively as ‘NOx’, carbon monoxide (CO), unburnt fuel or hydrocarbons and particulate matter.
In order to reduce or eliminate these pollutants, after-treatment systems typically contain the following components:
(i) A Diesel Oxidation Catalyst (DOC) to convert carbon monoxide to carbon dioxide (CO2) and unburnt fuel to water and CO2;
(ii) Diesel Particulate Filters (DPF) to reduce the amount of particulate matter, in particular soot and ash;
(iii) Selective Catalytic Reduction Catalysts (SCRC) to reduce NOx levels by converting NO and NO2 to nitrogen; and
(iv) Ammonia Slip Catalysts (ASC) to remove excess ammonia, which is injected into the exhaust system in the form of urea as part of the Selective Catalytic Reduction process.
The diesel particulate filters are typically wall flow filters which comprise a cylindrical ceramic monolith core (or another type of monolith) contained within a cylindrical metal surround or sleeve. The overall diameter of a filter for a vehicle exhaust system is generally between 200 and 400 mm. The ceramic filter cores have a number of elongate channels running along their length. These channels generally have an approximately square cross-section and are 1-2 mm in width. Between the channels the walls of the filter core are formed from a porous ceramic material. Neighbouring channels in the filter are plugged at alternate ends with ceramic material so that exhaust gases entering a channel at one end cannot exit the same channel at its other end. To exit, the gas must pass from one channel to the next through the porous walls of the filter. In doing so, soot and ash within the gas are deposited within and on the walls of the filter. The metal sleeve of the filter includes flanges at each end which, when in use, aid the connection of the filter to other parts of a vehicle's exhaust system.
The walls of the diesel particulate filters are typically coated with a catalyst washcoat in order to facilitate the process of regeneration, in which the soot deposits in the filter are periodically burnt away or oxidised from the filter. By contrast, the ash deposits continue to accumulate, since ash is non-combustible, and the diesel particulate filters, therefore, require periodic cleaning. Various known methods are used to clean the diesel particulate filters including air jets, compressed-air pulses and water-based processes.
In light-duty diesel engine applications (including cars, vans and other small vehicles), the ash accumulation process is relatively slow, and frequent cleaning of the diesel particulate filter is not required. However, in heavy-duty applications (such as in trucks, buses, excavators, tractors and the like) the use of the engine is much more intensive and diesel particulate filter cleaning is required on a regular basis, typically every 1 or 2 years depending on usage.
If the diesel particulate filter is removed from the vehicle for cleaning, various methods are used to ensure that the filter is in good condition before it is re-fitted to the vehicle. These include airflow tests and other inspection techniques. In addition to these physical tests, it is also desirable to measure the catalytic efficiency of the diesel particulate filter to ensure it is in satisfactory condition. The standard method for testing catalytic efficiency of a diesel particulate filter when it has been removed from the vehicle is to pass the exhaust gas from a stationary diesel engine through the filter and measure the levels of the relevant chemical components of the gas before and after the filter. An alternative to a stationary engine is a vehicle dynamometer arrangement in which the particulate filter is attached to a stationary test vehicle on a rolling-road device. U.S. Pat. No. 5,431,043 describes a method for testing the catalytic activity of a catalytic converter in an engine exhaust which involves running the engine to heat the converter to a normal operating temperature. Once the converter has reached the desired temperature the engine is shut off and the fuel and ignition systems of the engine are disabled and a source of hydrocarbon is introduced at a location upstream of the converter such as the air induction system. The engine is then cranked to pump the hydrocarbon through the system where it is mixed with air and to the converter. The exhausted constituents are analyzed and the activity of the converter is determined based on the levels thereof.
All of the methods use apparatus that is designed to mimic the use of the particulate filter in the vehicle exhaust system, but also permit the use of static exhaust gas analysers in a specially-equipped testing facility.
An important feature of these test methods is the speed of flow of exhaust gases through the test particulate filter. In order to make an accurate assessment of the catalytic activity of the filter, it is necessary to use a meaningful gas flow rate, substantially the same as the typical flow rates that would be experienced by the filter in use on the vehicle. Large diesel engines can produce in excess of 1 m3 of exhaust gas per second. The gas flow rate through the filter is typically measured in the form of a ‘space velocity’, which is related to the number of volume displacements through the filter in a given time. For example, a typical space velocity for a heavy duty vehicle may be in the region of 50,000-250,000 (volume displacements) per hour. This assumes a 20 liter filter can handle 1 cubic meter/sec; i.e. 50 changes/sec or 3000 changes per min or 180,000 changes per hour.
In order to replicate these space velocities during the testing of particulate filters used in heavy-duty applications, it is necessary to use a heavy-duty engine with a large capacity. However, the use of a large engine of this type means that typical testing facilities are complex and expensive, both in terms of installation and operating costs. There are also a number of other problems associated with traditional testing methods. Firstly, filters of different sizes and shapes require special adaptors to allow them to be connected securely into the exhaust line of the test engine. Secondly, the duration of the testing process is fairly long, owing to the time taken for the filter to reach the necessary temperatures to measure catalytic efficiency. These temperatures are typically in excess of 250° C. and the materials of the core of the filters generally have a large heat capacity and, therefore, can take a relatively long time to reach thermal equilibrium. Finally, the removal of the filter after testing is difficult because it is hot, usually in excess of 300° C., owing to the heat from the exhaust gases.
As a result, testing of catalytic activity is generally restricted to specialised development programs for new engines or exhaust systems, rather than the routine assessment of used parts.
It will be appreciated that although the foregoing description discussed testing of particulate filters, the same or similar methods are also commonly employed for testing other exhaust components such as DOCs, SCRCs and ASCs. As such, references in the following description to an exhaust component will be understood to include a number of different after-treatment systems including DPFs, DOCs, SCRCs and ASCs.
The object of this invention is, therefore, to provide an apparatus and method for testing exhaust components, which improves the speed, efficiency and cost of catalytic efficiency testing.